Not Applicable.
Not Applicable.
Not Applicable.
The present invention relates generally to resonance acceleration of charged particles and specifically to excitation of resonant cavities of a resonance accelerator with a radio-frequency electromagnetic wave. The invented device is a high power coupling loop which is used to interface a resonant cavity to a transmission line.
In its broadest classification, there are two types of charged particle accelerators; electrostatic, and resonance. In an electrostatic accelerator charged particles gain energy as they move between two regions that are held at two different electric potentials, for example two electrodes. Associated with the electric potential is the electric field E. The integral of the electric field along the path traversed by the particle that connects the two regions is equal to the potential difference between the two region. The accelerating force is equal to qE, where q is the electric charge of the particle. In an electrostatic accelerator, as the name xe2x80x9celectrostaticxe2x80x9d suggests, the electric potential is independent of time. Accelerating charged particles to megavolt energies requires a very large electric filed. The convenient unit in this context is megavolt/cm. Because both isolating materials and practical vacuum break down under strong electric fields the limit of the electrostatic acceleration is from several to around 10 MeV. The resonance accelerator, however, does not have this limitation.
Resonance accelerators like all electromagnetic devices owes their existence to the genius of J. C. Maxwell who added the displacement current to the Ampere""s law and implied new physical phenomena which has been substantiated in all details by experiment. Accordingly, a time varying magnetic fields give rise to electric fields and vice-versa. One of the device which is directly related to the present invention is the resonant cavity. In its simplest form a resonant cavity is a hollow volume enclosed by metallic walls. The hollow volume, as predicted by the Maxwell equations, supports electromagnetic oscillations in which the energy in the cavity oscillates between the electric and magnetic fields. In a resonance accelerator the particles are accelerated by electric field of the cavity or an array of cavities. Since both magnitude and direction of the electric field of a resonant cavity changes with time there must be an exact correlation between the movement of accelerating particles and the frequency of the resonant cavity: any time that the particles reaches the cavity field the electric field of the cavity should be in a direction to accelerate the particles. (The alternative is deceleration of charged particles which results in amplification of the cavity fields.) The term xe2x80x9cresonancexe2x80x9d in resonance accelerator refer to this requirement.
The resonant frequency of almost all cavities that are used for particle acceleration fall in the radio-frequency (rf for short) range. To resonate a cavity and keep it in the excited state the rf power from an rf amplifier must be continuously fed into the cavity. The transfer of power from the rf amplifier to the cavity is by a transmission line which connects the rf amplifier to the cavity. The end of the transmission line on the cavity side is connected to a coupling device which is housed inside the cavity. The coupling device interfaces the transmission line to the resonant cavity and plays a vital role in both operation of the accelerator and protection of the rf circuit elements.
From practical point of view the coupling device must possess some key features. First, the reflected rf power, the power that reflects back to the rf amplifier, should not exceed more than a few percent of the rf forward power. Here, the primary issue is not efficient use of the power but protection of the downstream componentsxe2x80x94the power amplifiers. A large amount of reflected power can easily ruin circuit elements on its pass. Second, the physical size of the coupling device should be much smaller that the physical size of the cavity. This condition warrants that the effect of the coupling device on the cavity is not more than a small perturbation. This requirement comes from the fact that in accelerator applications a resonant cavity with a large Q is desired. (The Q of the cavity is defined by
Q=xcfx89o Stored energy/Power loss
where xcfx89o is the angular frequency of the cavity.) A coupling device, however, reduces Q of the cavity. This reduction in Q gets worse as the physical size of the coupling device becomes larger.
With regard to the features of the present invention which will be discussed shortly, all existing high power coupling devices are nonadjustable and water cooled. The fact that they are nonadjustable means that many of them with different physical size and shape must be built and tried until one of them can provide a tolerable reflected power. The fact that they are water cooled means they are prone to leak water in the vacuum where they operate and the water cooling adds additional cost and maintenance. From these considerations it is highly desirable to come up with an adjustable coupling device that also does not use water cooling. Finally, a coupling device should be purely metallic. Any nonmetal part, such as ceramic or teflon, which is sometimes used for electrical isolation of a coupling device components will melt under high power.
The device to be described in the next section is the first adjustable high power rf coupling loop. Because it is adjustable it provides perfect impedance matching and subsequently renders almost zero reflected power. Moreover, it fulfills all other requirements discussed in this section; it is purely metallic, relatively small, and does not use water cooling. This coupling loop has been installed in an accelerator and has shown excellent performance.
A new coupling loop at radio frequency (rf) is presented which is used for interfacing a transmission line to a resonant cavity. All parts of the loop are metallic and subsequently the loop is ideal for high power rf applications. Its key parts comprises of two parallel metallic rods and a sliding clamps. One of the rod is connected to the center line of the transmission line and the other is hard soldered to the return; the ground. The two rods are shorted by the sliding clamp. The impedance matching is achieved by simply adjusting the position of the clamp. This is the first adjustable coupling loop and also the first loop that does not use water cooling. Since the loop is adjustable it can be adjusted to produce practically zero reflected power which is a highly desirable feature in resonance accelerators.